A transmission/scattering type optical element has been proposed in which a liquid crystal and a transparent polymer are combined to form a difference in the refractive index between the polymer and the liquid crystal or in the interior of the liquid crystal (among microscopic regions). It is called, for example, a liquid crystal/polymer composite element, a liquid crystal/resin composite element or a dispersion type liquid crystal element. In principle, this element requires no polarizing plate, whereby the light absorption loss is small, and a high scattering performance can be obtained, and thus, it has a significant merit in that the light utilizing efficiency in the entire element is high.
By utilizing such characteristics, it is employed for a light-controlling glass, an optical shutter, a laser apparatus and a display apparatus. One showing a scattering state under application of no voltage and a transparent state under application of a voltage, has been commercialized.
Further, prior art reference 1 (U.S. Pat. No. 5,188,760) discloses an element employing a liquid crystal and a polymerizable liquid crystal. According to this prior art reference 1, under application of no voltage, the element shows a transparent state as observed from any direction as the liquid crystal and the polymerized liquid crystals in the element have the same alignment direction, and under application of a voltage, the alignment of the liquid crystals in the element is controlled by an electric field, and the alignment direction of liquid crystal molecules changes variously in the microscopic regions, whereby the element shows a scattering state.
Further, it has been disclosed that the contrast ratio can be improved by adding a chiral agent to provide a helical structure in the initial alignment. This element is called “an anisotropic gel ” or “a liquid crystal gel”. In this prior art reference 1, a mesogen monomer having acryloyl groups at the terminals was used.
Further, prior art reference 2 (PCT International Publication WO92/19695) also discloses an element having a similar structure. The operation mode was the same as in prior art reference 1, and a very small amount of a polymer is dispersed in the chiral nematic liquid crystal to obtain a transparent state under application of no voltage and a scattering state under application of a voltage. This element is called PSCT (polymer stabilized cholesteric texture). Also in this prior art reference 2, a mesogen monomer having acryloyl groups at the terminals, was disclosed.
Characteristics of a liquid crystal optical device obtained by preparing a mixture comprising a liquid crystal and an uncured curable compound, and curing the curable compound to form a liquid crystal/cured composite layer, depend largely on the structure of the liquid crystal/cured composite. Further, the molecular structure of the uncured curable compound to be used, will give a substantial influence over the structure of the formed liquid crystal/cured composite.
It has been reported that in general, with a curable compound containing a mesogen structure such as a biphenyl structure, curable sites at both ends will bond, the modulus of elasticity after being cured is large, and the glass transition temperature of the obtainable polymer is also high.
On the other hand, this means that restrictions are given to the free volume and the molecular motion of the curable compound during the curing, and at the later stage of the curing process, the reactivity of the curable sites is likely to be suppressed, and there has been a problem that the curing reaction tends to be not enough, or a very long curing time will be required.
Namely, as the structures of uncured curable compounds, prior art reference 1 has disclosed the compound of the formula (2), and the prior art reference 2 has disclosed the compound of the formula (3); 
However, the cured products formed by using these compounds alone had characteristics attributable to the molecular structures. Namely, with the element disclosed in prior art reference 1, it is essential to use a monomer having a liquid crystal nature, and accordingly, the uncured curable compound contains a highly crystallizable rigid mesogen structural portion as shown in the formula (2), while with the compound of the formula (3) to be used in prior art reference (2), the distance between the mesogen structural portion and the curable site (an acryloyl group in this case) is short, and accordingly, the molecular weight between the crosslinking points tends to be small, whereby the obtained cured products tend to be hard and brittle. Further, for the same reason, the mobility of the uncured sites during the curing will be substantially impaired, whereby there has been a problem that a long curing time is required for adequate curing.
Further, with the liquid crystal optical elements of the prior art references, the voltage transmittance curves of the elements were likely to change by driving of application of an electric field for a plurality of times, and the contrast between application and non-application of an electric field, was still low. Particularly, the physical properties of the resin as the cured product of the liquid crystal/cured composite layer, related to the electro-optical characteristics of the liquid crystal optical element, and if the modulus of elasticity of the resin was too high or too brittle, the required driving voltage tended to be high, and at a relatively low driving voltage range, no adequate contrast ratio in the reflectance change or in the transmittance change was sometimes obtained between application and non-application of a voltage.
Further, as a means to improve the contrast ratio in the transmittance (or the reflectance) of a liquid crystal optical element obtainable between application and non-application of a voltage in the prior art, prior art reference 1 proposes to add a chiral agent to the mixture and to introduce a helical structure to the alignment mode of the curable compound after the curing, and prior art reference 2 proposes to add a chiral agent to bring the helical pitch to a level of from 0.5 to 4 μm.
However, the addition of the chiral agent may sometimes cause a problem such that it increases the driving voltage of the element or it decreases the transmittance when the element is transparent. Further, when a mixture of a liquid crystal with an uncured curable compound, is injected into a liquid crystal cell, or when it is sandwiched between substrates provided with transparent electrodes, such as resin films provided with electrodes, if the chiral agent is contained in a large amount, there will be a problem that injection irregularities or sandwiching irregularities are likely to result.
The present invention provides a liquid crystal optical element having high reliability and high contrast, whereby the voltage transmittance curve of the element will not substantially change even by driving of application or non-application of an electric field for a plurality of times. Further, it provides a production method whereby a liquid crystal optical element can be produced easily and constantly in good yield.
Further, it provides a liquid crystal optical element which can be produced, for example, in a short period of curing time and which has a high contrast ratio even at a low driving voltage.
Further, it provides a liquid crystal optical element, whereby addition of the chiral agent can be minimized, and a high contrast ratio can be obtained in the transmittance characteristics obtainable at the time of application and non-application of a voltage, and the driving voltage may be not increased as far as possible.